When an SSE route (or JSONL streaming route) is defined on an
`APIRouter` and merged onto a `FastAPI` app via `include_router`, the
merged route silently lost its `stream_item_type`. As a consequence the
emitted OpenAPI schema dropped the `contentSchema` describing the
streamed item and downstream tools (`datamodel-codegen`, etc.) could no
longer generate frame models from the spec.
Root cause: `APIRoute.__init__` only ran stream-item detection inside
the `isinstance(response_model, DefaultPlaceholder)` branch. The source
route's `__init__` collapses `self.response_model` to `None` after
detection, so when `APIRouter.include_router` re-instantiates the route
via `add_api_route(response_model=route.response_model, ...)` the new
init sees an explicit `None` and skips detection entirely.
Fix: also run detection when `response_model is None` on entry, and
restrict the "promote return annotation to response_model" fallback to
the original `DefaultPlaceholder` case so an explicit `response_model=
None` still disables response validation as documented.
Adds a regression test asserting both `stream_item_type` propagation and
the presence of `contentSchema` in the merged route's OpenAPI.
Co-Authored-By: Claude Code <[email protected]>
Jean-Baptiste Braun
eca7bc9602